Usually the scalp that once was covered by hair retains a slightly different texture and sheen that the skin of the forehead. If it is impossible to determine, you should place your tefillin above the point of where the hairline would be on an average person. If the bayis of the tefillin slides below the hairline you have not fulfilled the mitzva, but if just the wider base, titura, has slipped there is a machlokes. Generally speaking, it is better to place the Tefillin Shel Rosh a little too high rather than risking being too low.